For example, U.S. Pat. No. 7,129,176 corresponding to JP-A-2004-271756 has disclosed a technique for forming optical devices such as a microlens, a prism, and an optical waveguide on a silicon substrate. According to the technique disclosed in U.S. Pat. No. 7,129,176, first, a silicon substrate is etched by using a mask to form a number of trenches. As a result, multiple columnar members are separately arranged by the trenches. Each columnar member extends in parallel with a light axis so that transmittance of light can be improved. Then, the columnar members are thermally oxidized and changes to silicon oxide. As a result, the columnar members thermally expand, and the trenches are buried accordingly. Thus, a predetermined region of the silicon substrate becomes a block member. The block member exhibits an optical function depending on an outer shape thereof and acts as an optical device.
If an unoxidized portion (i.e., silicon) remains in the columnar members, the light transmittance of the block member is reduced, because the silicon is opaque to light. Therefore, it is important that the trenches exist between the columnar members to supply oxygen to the columnar members until the columnar members are completely oxidized. It has been empirically known that a thermal oxidation layer grows outside and inside with respect to a surface of the silicon substrate by a ratio of 0.55:0.45, respectively.
The unoxidized portion remaining in the columnar members after the thermal oxidation can be prevented by increasing width of the trenches. In this case, however, the trenches cannot perfectly buried after the columnar members expand due to the thermal oxidation. As a result, gaps derived from the trenches remain between the columnar members after the thermal oxidation. Since the gaps are regularly arranged, the gaps serves as a diffractive grating. Therefore, the light passing through the block member is divided into order diffraction lights traveling in different directions. The optical device uses only one of the order diffraction lights. For example, when the optical element acting as a lens, a zero-order diffraction light is used. As a result, the light transmission of the optical device is substantially reduced.